1. Field of the Invention
The invention relates to the art of tire inflation systems. More particularly, the invention relates to tire inflation systems for heavy-duty vehicles such as tractor-trailers or semi-trailers, which can operate as the vehicles are moving.
2. Background Art
All tractor-trailers include at least one trailer, and sometimes two or three trailers, all of which are pulled by a single tractor. Each trailer typically includes eight or more tires, each of which is inflated with air. Optimally, each tire is inflated to a recommended pressure that is usually between about 70 pounds per square inch (psi) and about 130 psi. However, it is well known that air may leak from a tire, usually in a gradual manner, but sometimes rapidly if there is a problem with the tire, such as a defect or a puncture caused by a road hazard. As a result, it is necessary to regularly check the air pressure in each tire to ensure that the tires are not under-inflated. Should an air check show a tire that is under-inflated, it is desirable to enable air to flow into the tire to return it to an optimum tire pressure.
The large number of tires on any given trailer setup makes it difficult to manually check and maintain the optimum tire pressure for each and every tire. This difficulty is compounded by the fact that multiple trailers in a fleet may be located at a site for an extended period of time, during which the tire pressure might not be checked. Any one of these trailers might be placed into service at a moment's notice, leading to the possibility of operation with under-inflated tires. Such operation may increase the chance of failure of a tire in service as compared to operation with tires in an optimum inflation range.
Moreover, should a tire develop a leak, for example, as a result of striking a road hazard, the tire could fail if the leak continues unabated as the vehicle travels over-the-road. The potential for tire failure often is more pronounced in vehicles such as tractor-trailers that travel for long distances and/or extended periods of time.
As a result of such problems, prior art systems were developed that attempt to automatically monitor the pressure in a vehicle tire and/or inflate the vehicle tire with air to a minimum tire pressure as the vehicle is moving. Many of these automated systems utilize rotary unions that transmit air from a pressurized axle or air line to the rotating tires. These prior art systems either are constantly pressurized or use an intermittent pressure check-and-fill procedure. However, these prior art systems exhibit several disadvantages.
Rotary unions that are constantly pressurized enable a simple mechanical air pressure regulator to set the tire pressure. Such systems typically utilize a flow switch to warn of low tire pressure, a leaking line or a punctured tire. However, such systems generally also can give false positive warnings. For example, simply filling the air lines may cause a sensor to give a false positive warning. Moreover, constantly pressurized rotary unions have high contact pressure at the sealing point of the rotary union seals, which limits the useful life of the rotary union.
Systems which utilize intermittent pressurization of the rotary union dramatically reduce the time that the rotary union seals are under pressure, thereby typically increasing the life of the rotary union. However, such intermittent-type systems generally require some type of electronic control which includes simple solenoid valves and a pressure-measuring device. Some of these systems also require a personal computer (PC) to be interfaced to the electronic controller to program tire pressure settings. However, access to PC's, the proper interface cables and interface modules often are not readily available in the field, creating problems when the tire pressure setting is to be changed. Other intermittent-type systems are preprogrammed with a self-learn mode that does not require the PC interface. However, such systems require each tire on a given trailer to be manually inflated, which is problematic since many original equipment manufacturers of trailers do not have consistent shop air pressure to enable uniform tire inflation, particularly on higher-inflation pressure tires. Consequently, the self-learn mode sets to the lowest tire pressure, which can be significantly less than optimal.
In addition, constantly pressurized rotary union systems and intermittently pressurized rotary union systems include check valves between the air supply and each tire. These check valves in effect isolate each tire by allowing air to flow into the tire but not out. Moreover, in intermittently pressurized rotary union systems, check valves hold the air in each respective tire when the system is not pressurized. However, if the control systems of the prior art tire inflation systems detect a failure or malfunction of a check valve, they do not compensate by maintaining pressure in the delivery lines, thereby allowing a tire to deflate should a respective check valve malfunction.
Moreover, rotary unions used in prior art tire inflation systems include a single-piece body construction that prevents servicing of the rotary union, as well as multiple-piece rigid air tubes that could fail at the joint between the tubes. These prior art rotary unions also have a means of attachment to the axle that is not optimum for long-term use and hose barb fittings that potentially can allow air hoses to work loose over time. With such characteristics, these prior art rotary unions thus are potentially susceptible to premature failure, which is highly undesirable.
As a result, the tire inflation systems of the prior art include significant disadvantages by not providing reliable automatic control over the inflation process, failing to keep the system pressurized in the event of malfunction of check valve, failing to communicate system problems, and potentially lacking long-term rotary union stability. Therefore, a longstanding need has existed in the art for a tire inflation system that provides for more extensive monitoring and more reliable control of the tire inflation process, communication of system problems without a PC interface, improved mechanical stability of the rotary union, and an ability to maintain air pressure if a check valve fails.